Function

Functions are C++ entities that associate a sequence of?statements ?(a?function body) with a?name?and a list of zero or more?function parameters.#article

// function name: "isodd"
// parameter list has one parameter, with name "n" and type int
// the return type is bool
bool isodd(int n)
{                      // the body of the function begins
    return n % 2;
}                      // the body of the function ends        

When a function is invoked, e.g. in a?function-call expression , the parameters are initialized from the arguments (either provided at the place of call or?defaulted ) and the statements in the function body are executed. If the?parameter list ?ends with?..., extra arguments can be supplied to the function, such a function is called?variadic function .

int main()
{
    for(int arg : {-3, -2, -1, 0, 1, 2, 3})
        std::cout << isodd(arg) << ' '; // isodd called 7 times, each
                                        // time n is copy-initialized from arg
}        

Unqualified ?function names in function-call expressions are looked up with an extra set of rules called?"argument-dependent lookup" (ADL) .

A function can terminate by?returning ?or by?throwing ?an?exception .

A function may be a?coroutine , in which case it can suspend execution to be resumed later.

(since C++20)

A?function declaration ?may appear in any scope, but a?function definition ?may only appear in namespace scope or, for?member ?and?friend ?functions, in class scope. A function that is declared in a class body without a friend specifier is a class member function. Such functions have many additional properties, see?member functions ?for details.

Functions are not objects: there are no arrays of functions and functions cannot be passed by value or returned from other functions. Pointers and references to functions (except for?the main function ?and?most standard library functions ?(since C++20)) are allowed, and may be used where these functions themselves cannot. Therefore we say these functions are "addressable".

Each function has a type, which consists of the function's return type, the types of all parameters (after array-to-pointer and function-to-pointer transformations, see?parameter list )?, whether the function is?noexcept ?or not?(since C++17), and, for non-static member functions, cv-qualification?and ref-qualification?(since C++11). Function types also have?language linkage . There are no cv-qualified function types (not to be confused with the types of?cv-qualified functions ?such as?int?f()?const;?or functions returning?cv-qualified types , such as?std::string ?const?f();). Any cv-qualifier is ignored if it is added to an alias for a function type.

Unnamed functions can be generated by?lambda-expressions .

(since C++11)

Multiple functions in the same scope may have the same name, as long as their parameter lists and, for non-static member functions, cv/ref?(since C++11)-qualifications are different. This is known as?function overloading . Function declarations that differ only in the return type?and the noexcept specification?(since C++17)?cannot be overloaded. The?address of an overloaded function ?is determined differently.

Function objects

Besides function lvalues, the function call expression supports pointers to functions, and any value of class type that overloads the function-call operator or is convertible to function pointer?(including?lambda-expressions )?(since C++11). Together, these types are known as?FunctionObjects , and they are used ubiquitously through the C++ standard library, see for example, usages of?BinaryPredicate ?and?Compare .

The standard library also provides a number of pre-defined?function object templates ?as well as the methods to compose new ones (including?std::less ,?std::mem_fn ,?std::bind ,?std::function ?(since C++11), and?std::bind_front ?(since C++20), and?std::move_only_function?(since C++20)).